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RE: Chapter 46: preamble length


Preamble originally existed to provide for the needs of the physical layer.
At the original speed, the physical layer components needed to receive some
preamble bits before they removed their squelch and the receiving PLS
(roughly equivalent to our PCS) had to receive enough preamble to lock its
clock before it could receive data. Therefore, all the PHY components
consumed preamble bits and the preamble length was partly to serve this
purpose and partly to allow Sending End Overshoot to die out on the coax
before data started. At 10 Mbits/s, not only can preamble shrink, but
repeaters can add preamble bits.

It is only the higher speed PHYs that allow us the option of preserving the
preamble. In doing 1 Gbit/s, we decided to allow the Phy to delete preamble
to align the start delimiter.  Since a start could appear on even
characters, this only required deleting one character. At 10 Gig, it would
require deleting 3 characters of preamble and we decided it would be better
to change the IPG and preserve the full preamble. 


-----Original Message-----
From: Sanjeev Mahalawat [mailto:sanjeev@xxxxxxxxx]
Sent: Tuesday, March 27, 2001 6:08 PM
To: Grow, Bob; 'Danielle Lemay'; stds-802-3-hssg@xxxxxxxx
Subject: RE: Chapter 46: preamble length

Hi Bob,

At 02:21 PM 03/27/2001 -0800, Grow, Bob wrote:
>On transmit, a conforming implementation will send seven preamble plus the
>On receive, there is no current function that will change that length, but
>the concensus of the committee was to keep the option open.  (In 802.3z we
>did change preamble length for idle alignment.)  The D3.0 text should make
>it clear that an implementation should be tolerant to changes in preamble
>length, though it can still rely on lane alignment (Start in lane 0, SFD in
>lane 3).  Text was added to warn that the Start and SFD could appear in the
>same column.

What is the reasoning behind letting a layer lower than
MAC to touch the preamble?

Since preamble is coded as data it belongs to MAC 
and no lower layer should be allowed to change 
and/or remove the length of preamble.


>--Bob Grow
>-----Original Message-----
>From: Danielle Lemay [mailto:dlemay@xxxxxxxxxxxxxxxxx]
>Sent: Monday, March 12, 2001 10:38 AM
>To: stds-802-3-hssg@xxxxxxxx
>Subject: Chapter 46: preamble length
>Is it possible for the preamble+SFD to be less than 8 bytes ?
>Danielle Lemay
>Design Engineer, Nishan Systems